Injection compression molding method for optically formed product

ABSTRACT

A method of producing an optically molded product of a thermoplastic resin by injection compression molding, the method comprising the steps of: 
     (1) expanding the volume of a cavity more than the volume of the optically molded product of interest; 
     (2) injecting a molten thermoplastic resin into the cavity through an injection cylinder; 
     (3) compressing the expanded cavity to a prescribed thickness of the molded product; 
     (4) returning an excess of the thermoplastic resin produced by the compression into the injection cylinder; and 
     (5) keeping the molten thermoplastic resin in the cavity until the molded product of interest is formed. The present invention makes it possible to produce an optically molded product (for example, a spectacle lens) free from a weld line and excellent in surface accuracy using a metal mold having a simple structure industrially advantageously.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of producing an opticallymolded product by injection compression molding. More specifically, itrelates to a method of producing an optically molded product which isrequired to have excellent surface accuracy and optical properties, suchas a spectacle lens, from a transparent thermoplastic resin such as apolycarbonate resin by injection compression molding.

PRIOR ART

Demand for plastic lenses has recently been growing. Plastic lenses areroughly divided into acrylic resin lenses produced bycasting-thermosetting molding and thermoplastic resin lenses such aspolycarbonate resin and polyacrylic resin lenses produced by injectionmolding.

Particularly, polycarbonate resin lenses have been attracting muchattention as spectacle lenses which can be produced by injection moldingand have a high refractive index, light weight, excellent ultravioletlight absorptive power and safety with impact resistance and have beenused recently in large quantities.

There are proposed a large number of methods of producing spectaclelenses by injection molding. Known methods include one in which asemi-finish lens is produced by injection molding and shaped into anoptical form of interest by cutting and polishing and one in which alens having an optical form is obtained by a single injection moldingprocess as a finish lens. Particularly, the latter case involves a basicproblem caused by the form of a lens that when a concave lens is to beproduced by injection molding, a molten resin from a gate flows fast ina portion corresponding to the peripheral portion of the lens because itis thick and slow in a portion corresponding to the center portion ofthe lens because it is thin in a cavity.

As a result, resin flows meet each other in the peripheral portion inthe end, forming a weld line. This is marked as the focusing distance ofthe concave lens becomes shorter, thereby deteriorating the opticalproperties of the lens and greatly impairing the appearance of the lens.Depending on the outer diameter of a lens, the formation of the weldline which is a quality problem rarely occurs in lenses having afocusing distance of about −1 m or less. However, the formation of theweld line easily occurs in concave lenses having a focusing distance of−1 m or more, particularly concave lenses having a focusing distance of−0.5 m or more.

There are proposed various methods of solving the above problem causedby the injection molding of spectacle lenses. Out of the methods,typical methods for eliminating the weld line include one in which anoverflow pocket is formed in a peripheral portion to limit a resin flowin the peripheral portion relatively as disclosed by JP-B 61-19409 (theterm “JP-B” as used herein means an “examined Japanese patentpublication”), one in which a side core is formed in a cavity asdisclosed by JP-A 62-83121 (the term “JP-A” as used herein means an“unexamined published Japanese patent application”), and one in whichvery small irregularities are formed on the most peripheral portion of alens to limit a resin flow in the peripheral portion as disclosed byJP-A 1-90716.

However, when the overflow pocket or the side core is formed in thecavity, the structure of a metal mold becomes complex, the peripheralportion of a powerful concave,lens (may be referred to as “minus lens”)becomes very thick, and a weld line cannot be eliminated substantiallyin these methods.

Since a flow of a molten resin in the cavity is disturbed in the methodsin which the side core or irregularities are formed in the cavity, amolding defect such as a flow mark or a cold flow is easily induced andthe releasability of a lens molded product is impaired. A cylindricalsurface or toric surface on the most peripheral portion of which changesin thickness cannot be formed with these methods.

Further, as the production of a spectacle lens by injection molding isgenerally carried out by a method of molding a great quantity andmultiple types of spectacle lenses at the same time, it is verycomplicated to attach and detach accessories to and from the cavity,thereby greatly limiting the type of spectacle lenses which can bemolded.

Although a multi-stage compression method makes it possible to obtain alens having excellent surface accuracy, the formation of a weld linecannot be prevented basically because the amount of a resincorresponding to the amount of shrinkage is filled.

Thus, there are proposed a large number of methods of preventing theformation of a weld line, such as one in which the temperature of ametal mold is controlled in a complicated manner and one making use ofultrasonic waves. However, these methods involve such problems as thecomplex structure of a metal mold and the limited design of a lens. Amethod of molding an optical lens having substantially no weld line isnot yet to be established.

When a finish lens is to be produced by injection molding, a distortionor a reduction in surface accuracy is easily caused by the shrinkage ofa resin due to solidification by cooling. This phenomenon is more markedas the shrinkage difference of a molded product having a large thicknessdifference becomes larger. To eliminate a shrinkage difference caused bysolidification by cooling, there is a typical one as disclosed by JP-B6-71755 in which a multi-stage compression method is carried on whilethe amount of a resin equivalent to the amount of shrinkage is filled inadvance. However, this method has such problems as insufficient surfaceaccuracy and large differences among molded products. The expression“surface accuracy” as used herein means whether surface curvature,flatness and the like fall within designed standard ranges.

These known injection compression molding methods have the following twodefects. One of the defects is that a sufficient injection resinpressure is not applied at the time of the completion of injection whenthe surface layer of a molded product which exerts a great influenceupon the surface accuracy and optical distortion of the molded productis formed because a resin is filled into a cavity and a sufficientinjection resin pressure is not applied at the time of injection.Therefore, a distortion or poor surface accuracy results. The expression“optical distortion” as used herein means an optical distortion whichcan be easily observed with the naked eye, an optical distortion whichcan be observed from an image of a fluorescent light reflected on thesurface of a lens, an optical distortion which can be seen as a thinring form by a polarizing plate, or the like. These defect phenomena canbe easily discovered the most by observation with a polarizing plate.These are fatal defects when the molded product is used as a lens. Thesedefect phenomena easily occur in the central portion of a plus lens andthe peripheral portion of a minus lens. The cause of these phenomena ismainly that a sufficient resin pressure cannot be applied to the insideof the cavity at the time of the completion of injection when thesurface layer of a lens is formed.

The second defect is that the filled state of a resin in the injectionstep of an injection molding machine includes differences in theinjection step or differences in the metering step and the state of aresin in the cavity before compression often greatly differs for eachmolding shot. Therefore, there often produced differences above atolerable range from the viewpoint of the quality control of anoptically molded product. The injection compression molding of the priorart is greatly affected by the differences.

Problem to be Solved by the Invention

It is therefore a first object of the present invention to provide amethod of molding an optically molded product which has no weld line ora very small weld line if it is formed.

It is a second object of the present invention to provide a method ofmolding an optically molded product having excellent surface accuracy.

It is a third object of the present invention to provide a method ofmolding an optically molded product, which facilitates molding multiplekinds and a large quantity of molded products at the same time withoutusing a cavity having a complex shape.

It is another object of the present invention to provide a method ofmolding an optically molded product, which has small differences ofquality molded products and which can mold a high-quality molded productstably and industrially advantageously.

It is a further object of the present invention to provide a method ofmolding an optically molded product economically advantageously.

Means for Solving Problems

According to studies conducted by the inventors of the presentinvention, the above objects of the present invention are attained by amethod of producing an optically molded product of a thermoplastic resinby injection compression molding, the method comprising the steps of:

(1) expanding the volume of a cavity more than the volume of theoptically molded product of interest;

(2) injecting a molten thermoplastic resin into the cavity through aninjection cylinder;

(3) compressing the expanded cavity to a prescribed thickness of themolded product;

(4) returning an excess of the thermoplastic resin produced by thecompression into the injection cylinder;

(5) keeping the molten thermoplastic resin in the cavity until themolded product of interest is formed; and

(6) taking out the obtained molded product from the cavity.

This molding method will be referred to as “first molding method”hereinafter.

Further, according to studies conducted by the inventors of the presentinvention, the above objects of the present invention are attained by amethod of producing an optically molded product of a thermoplastic resinby injection compression molding, the method comprising the steps:

(1) expanding the volume of a cavity more than the volume of theoptically molded product of interest;

(2) injecting a molten thermoplastic resin into the cavity through aninjection cylinder;

(3) compressing the expanded cavity to a prescribed thickness of themolded product or a thickness 200 p2m smaller than the thickness;

(4) adjusting or changing resin pressure in the injection cylinder andcompression pressure in the cavity in limits that the change width doesnot exceed 100 μm from the prescribed thickness of the molded product toachieve the prescribed thickness of the molded product in the end;

(5) keeping the molten thermoplastic resin in the cavity until themolded product of interest is formed; and

(6) taking out the obtained molded product from the cavity.

This molding method will be referred to as “second molding method”hereinafter.

The expression “optically molded product” as used herein denotes anoptically molded product obtained by forming an image of an object usinglight refraction and reflection and diverging or converging a pencil ofrays, or an optically molded product obtained making use of aninterference phenomenon or divergence caused by the phase difference oflaser beams. Illustrative examples of the optically molded productinclude plastic spectacle lenses and projector lenses. The presentinvention is particularly advantageous for the molding of apolycarbonate resin spectacle lens.

Since a resin is shut off from a runner by such means as a gate seal andthen a cavity is compressed in the injection compression molding methodof the prior art, an excess of the resin is stored in an overflow pocketor the like. Since a small amount of the filled resin for making up forthe amount of shrinkage is existent, a complex metal mold structure isrequired for inhibiting the forming of weld line to provide a side coreor irregularities, control the temperature of a metal mold or make anultrasonic treatment.

The inventors of the present invention ventured to study means ofreturning a molten resin into the injection cylinder, which has beenconsidered inappropriate in the prior art injection molding technology,and could establish an injection compression molding method which canhandle a small excess or large excess of the resin with very simplemeans and rarely forms a weld line for a lens having any shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a cavity which is expanded morethan the volume of an optically molded product before injection;

FIG. 2 is a schematic diagram showing that a molten thermoplastic resinis injected into the expanded cavity;

FIG. 3 is a schematic diagram showing that the expanded cavity iscompressed to a prescribed thickness (center thickness) to return anexcess of the molten thermoplastic resin into an injection cylinder;

FIG. 4 is a conceptual diagram of the injection compression moldingmethod of the prior art;

FIG. 5 is a conceptual diagram of the injection compression moldingmethod of the present invention;

FIG. 6 is a diagram showing the relationship among the amount ofcompression, compression pressure and resin pressure;

FIG. 7 is a diagram showing the relationship among the amount ofcompression, compression pressure and resin pressure when the cavity isexcessively compressed by 20 to 200 μm more than the prescribedthickness and returned to the prescribed thickness by controlling resinpressure and compression pressure;

FIG. 8 is a diagram showing the relationship among the amount ofcompression, compression pressure and resin pressure when resin pressureis increased and compression pressure is reduced in multiple stagesalong with the cooling step;

FIG. 9 is a schematic diagram showing the positional relationship amongthe eye, sample and fluorescent lamp at the time of the evaluation ofthe fluorescent lamp;

FIG. 10 is a schematic diagram showing the positional relationshipbetween the structure of a distortion inspection device and a sample;and

FIG. 11 is a diagram typically showing a lens having a weld line.

REFERENCE SYMBOLS

A. expanded cavity

B. mirror surface on fixed side

C. mirror surface on movable side

D. molten resin

E. screw

F. compressed cavity

1. mirror surface on fixed side

2. mirror surface on movable side

3. cavity

4. compression control rod

5. die set on fixed side

6. space having a width equivalent to amount of compression

7. ejector plate

8. pedestal

9. compression plate

10. compression force

11. counter force

12. magnetic scale

13. resin pressure

14. amount of compression

15. compression pressure

16. resin pressure

17. compression pressure time in compression step

18. resin pressure time in compression step

19. time of compression step

20. resin pressure in compression step

21. compression pressure in compression step

22. resin pressure in dwelling step

23. compression pressure in dwelling step

24. change width in amount of compression after compressed topredetermined thickness

25. amount of return from compression

26. returning step

27. resin pressure multi-stage control unit

28. injection pressure multi-stage control unit

29. positional relationship between eye and sample for evaluation offluorescent lamp

30. fluorescent lamp

31. polarizing plate whose polarization planes cross each other

32 sample (optically molded product)

33. frosted glass

34. fluorescent lamp

35. sample (minus lens)

36. length of weld line

37. time of dwelling step

The molding method of the present invention will be described in moredetail hereinunder.

The injection molding machine used in the present invention is notparticularly limited but it has required clamping force as the basis ofthe injection molding of an optically molded product and desirably has aunit capable of controlling injection, compression and dwelling inmultiple stages with high accuracy. The screw unit may have any shape ifit has a counter-flow prevention unit. This molding machine may be ofany type such as an in-line screw type or plunger type.

The metal mold used in the present invention is not particularly limitedif it can be used for compression molding. It can be used for a clampingcompression method making use of the opening and closing of a platen(plate for attaching a metal mold) or a core compression method usingthe compression cylinder of the platen of a molding machine or ballscrew. The core compression method is preferred because the cavity mustbe opened wide to completely eliminate a weld line.

The clamping compression method is a method in which the partingsurfaces of a fixed mold and a movable mold are separated from eachother with a predetermined interval therebetween to open a metal mold, aresin is injected, the parting surfaces are contacted to each other byclamping force, and a cavity is compressed. The core compression methodis a method in which the parting surfaces of a fixed mold and a movablemold are contacted to each other by clamping before injection, a resinis injected by applying predetermined clamping force, and then a cavityis compressed. In the compression step after injection, a mirror surfaceon the movable side is moved forward in a direction that the volume ofthe cavity is reduced for compression by means of a compression unitinstalled in a molding machine, the metal mold or the like. Thecompression unit is a hydraulic cylinder, ball screw or the like.

In the molding method of the present invention, as a complex unit suchas an overflow pocket or side core does not need to be installed in themetal mold, a metal mold having a very simple structure can be used.

The thermoplastic resin used in the present invention is a transparentresin such as a polycarbonate resin, polyacrylic resin or modifiedpolyolefin resin. Out of these, a polycarbonate resin is the mostpreferred as a raw material for optically molded products, particularlyspectacle lenses.

The polycarbonate resin which can be used in the present invention isobtained by interfacial polymerization or ester interchange and has aviscosity average molecular weight of 17,000 to 40,000, preferably20,000 to 30,000. Since optically molded products, particularlyspectacle lenses are precision molded products, it is important toprovide a prescribed curvature and diopter by transferring the mirrorsurface of a metal mold accurately, and a resin having low viscositywith excellent melt flowability is desired. However, if the viscosity ofthe polycarbonate resin is too low, impact strength which is thecharacteristic property of the polycarbonate resin cannot be retained.The expression “viscosity average molecular weight (M)” as used hereinis obtained from the following Schnell's equation based on the intrinsicviscosity [η] of a solution measured at 20° C. in a methylene chloridesolvent using an Ostwald's viscometer.

[η]=1.23×10⁻⁴M^(0.83)

A bisphenol used to produce the polycarbonate resin is particularlypreferably bisphenol A. However, polycarbonate resins obtained bypolymerizing other known phenols are acceptable.

The polycarbonate resin used in the present invention is an aromaticpolycarbonate resin obtained by reacting a diphenol and a carbonateprecursor. Illustrative examples of the diphenol includebis(hydroxyaryl)alkanes such as 2,2-bis(4-hydroxyphenyl)propane(so-called bisphenol A), bis(4-hydroxyphenyl)methane,1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)butane,2,2-bis(4-hydroxyphenyl)octane, 2,2-bis(4-hydroxyphenyl)phenylmethane,2,2-bis(4-hydroxy-3-methylphenyl)propane,1,1-bis(4-hydroxy-3-tert-butylphenyl)propane,2,2-bis(4-hydroxy-3-bromophenyl)propane,2,2-bis(4-hydroxy-3,5-dibromophenyl)propane and2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane;bis(hydroxyphenyl)cycloalkanes such as1,1-bis(hydroxyphenyl)cyclopentane and 1,1-bis (hydroxyphenyl)cyclohexane; dihydroxyaryl ethers such as 4,4′-dihydroxydiphenyl etherand 4,4′-dihydroxy-3,3′-dimethyldiphenyl ether; dihydroxydiaryl sulfidessuch as 4,4′-dihydroxydiphenyl sulfide and4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfide; dihydroxydiaryl sulfoxidessuch as 4,4′-dihydroxydiphenyl sulfoxide and4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfoxide; and dihydroxydiarylsulfones such as 4,4′-dihydroxydiphenyl sulfone and4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfone. These diphenols may beused alone or in combination of two or more.

Preferably, the polycarbonate resin contains2,2-bis(4-hydroxyphenyl)propane (bisphenol A), out of the abovediphenols, as the main diphenol component. Particularly preferably, thepolycarbonate resin contains bisphenol A in an amount of 70 mol % ormore, particularly 80 mol % or more based on the total of all thediphenol components. The most preferred is an aromatic polycarbonateresin whose diphenol component is substantially composed of bisphenol A.

A brief description is given of interfacial polymerization or esterinterchange for the production of the polycarbonate resin. In theinterfacial polymerization in which phosgene is used as the carbonateprecursor, a reaction between a diphenol component and phosgene isgenerally carried out in the presence of an acid binder and an organicsolvent. An alkali metal hydroxide such as sodium hydroxide or potassiumhydroxide, or an amine compound such as pyridine is used as the acidbinder. A hydrocarbon halide such as methylene chloride or chlorobenzeneis used as the organic solvent. A catalyst such as a tertiary amine orquaternary ammonium salt may be used to promote the reaction. A terminalcapping agent such as an alkyl-substituted phenol exemplified by phenolor p-tert-butylphenol is desirably used as a molecular weight controlagent. The reaction temperature is generally 0 to 40° C., the reactiontime is several minutes to 5 hours, and pH is preferably maintained at10 or more during the reaction.

The ester interchange (melting method) using a carbonic acid diester asthe carbonate precursor is to distill out an alcohol or phenol formed bystirring under heating a diphenol component and a carbonic acid diesterin a predetermined ratio in the presence of an inert gas. The reactiontemperature which differs according to the boiling point of the formedalcohol or phenol is generally 120 to 330° C. The reaction is carriedout while the alcohol or phenol formed by reducing the pressure from thebeginning of the reaction is distilled out. A general ester interchangereaction catalyst may be used to promote the reaction. The carbonic aciddiester used for the ester interchange reaction is, for example,diphenyl carbonate, dinaphthyl carbonate, dimethyl carbonate, diethylcarbonate, dibutyl carbonate or the like, out of which diphenylcarbonate is particularly preferred.

A parting agent may be blended into the polycarbonate resin of thepresent invention, which provides a preferred result. The parting agentis generally a saturated fatty acid ester such as a monoglycerideexemplified by monoglyceride stearate, low fatty acid ester exemplifiedby stearil stearate, higher fatty acid ester exemplified by behenatesebacate or erythritol ester exemplified by pentaerythritoltetrastearate. The parting agent is used in an amount of 0.03 to 1 partby weight based on 100 parts by weight of the polycarbonate resin. Aphosphorous acid ester-based heat stabilizer may be used in an amount of0.001 to 0.1 part by weight based on 100 parts by weight of thepolycarbonate resin as required. The phosphorous acid ester-based heatstabilizer is preferably tris(nonylphenyl)phosphite, triphenylphosphite, tris(2,4-di-tert-butylphenyl)phosphite,tetrakis(2,4-di-tert-butylphenyl)-4,4′-biphenylene diphosphonite,bis-(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol-di-phosphite,bis(2,4-di-tert-butylphenyl)pentaerythritol-diphosphite,tris(ethylphenyl)phosphite, tris(butylphenyl)phosphite ortris(hydroxyphenyl)phosphite, particularly preferablytris(nonylphenyl)phosphite ortetrakis(2,4-di-tert-butylphenyl)-4,4′-biphenylene diphosphonite.

An ultraviolet light absorber may further be blended into thepolycarbonate resin of the present invention to improve weatherabilityand cut harmful ultraviolet light. The ultraviolet light absorber is,for example, a benzophenone-based ultraviolet light absorber typified by2,2′-dihydroxy-4-methoxybenzophenone, or benzotriazole-based ultravioletlight absorber typified by2-(3-tert-butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzotriazole,2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole,2,2′-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol],2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole or2-(3,5-di-tert-amyl-2-hydroxyphenyl)benzotriazole. They may be usedalone or in combination of two or more. Out of these ultraviolet lightabsorbers, benzotriazole-based ultraviolet light absorbers arepreferred.

A bluing agent may further be blended into the polycarbonate resin ofthe present invention to cancel the yellow tint of a lens based on thepolycarbonate resin and the ultraviolet light absorber. Any bluing agentmay be used without a problem if it is used for a polycarbonate resin.An anthraquinone-based dye which is easily acquired is generallypreferred.

The first molding method of the present invention will be described withreference to FIGS. 1 to 3.

FIG. 1 is a schematic diagram showing a cavity A which has been expandedmore than the volume of an optically molded product before injection,FIG. 2 is a schematic diagram showing that a molten thermoplastic resinis injected into the expanded cavity A, and FIG. 3 is a schematicdiagram showing that the expanded cavity is compressed to a prescribedthickness (center thickness) to return an excess of the moltenthermoplastic resin into an injection cylinder.

Since a weld line is formed on an optically molded product having athickness difference, attention has been paid to the ratio of thesmallest thickness to the largest thickness of an optically moldedproduct, for example, the thickness of the center portion (smallestthickness) to the thickness of the peripheral portion (largestthickness) of a concave lens in the present invention. When the ratio ofthe smallest thickness to the largest thickness which is given by thefollowing equation (100×largest thickness/smallest thickness) is 150% ormore, particularly more than 300% though it differs according to theshape of an optically molded product, a weld line is formed.

When an excess of the resin is returned into the injection cylinderafter the cavity is expanded before injection to such a limit level thata weld line is rarely formed, that is, the above ratio becomes smallerthan 150%, preferably 140% or less and then filled with a molten resin,an optically molded product, for example, a lens almost free from a weldline is obtained even when it is a concave lens having a short focusingdistance. The return of the resin into the cylinder can be confirmedfrom an increase in the measurement value of an injection strokemeasuring instrument attached to an injection unit in a directionopposite to the injection direction of the resin.

The compression amount of the cavity, which differs according to moldingconditions such as the type and design value of typical thickness(thickness of a center portion in the case of a concave lens) of anoptically molded product, the temperature of the cylinder and thetemperature of the metal mold, the molding machine used, the metal moldand the like must be made larger as the focusing distance of a concavelens which is a typical example of the optically molded product becomesshorter. As for the volume of the expanded cavity at the time ofinjection based on the volume of the lens, the expansion volume ratiocalculated from the following equation is preferably in the range of 110to 500% from the viewpoints of the surface accuracy (surfacedeformation, etc.), optical properties (focusing distance, aberration,etc.) and molding ease of an optically molded product. It is morepreferably in the range of 120 to 400%, particularly preferably 150 to350%. When the expansion volume ratio is more than 500%, the amount ofthe resin to be exhausted becomes large, whereby required compressionpressure may rise, the heat resistance of the molten resin maydeteriorate or a molding failure may occur. The amount of compressioncaused by compression denotes a difference in typical thickness betweenthe optically molded product before compression and the optically moldedproduct after compression.

expansion volume ratio (%)=100×(volume of expanded cavity/volume ofcompressed cavity)

The volume of the expanded cavity and the volume of the compressedcavity are expressed in the unit of ml.

The preferred range of the expansion volume ratio is also affected bythe above thickness ratio when a concave lens is molded. For example,the expansion volume ratio is preferably in the range of 110 to 200%when the thickness ratio is small (for example, 300% or less) and 200 to500% when the thickness ratio is larger than that.

In the molding method of the present invention, the cavity must beexpanded to such a limit level that a weld line is not formed or isallowable in a molded product before the completion of the injectionstep before compression. It is difficult to completely eliminate a weldline formed before compression by any means. That is, the cavity must beexpanded before injection to such a limit level that the weld line isnot formed or is allowable.

When the cavity is expanded to a predetermined thickness and a lens iscooled without compression and taken out from the cavity, it can bechecked whether a weld line is formed before compression. As the amountof compression is smaller, molding becomes easier, and it is preferredthat the amount of compression can be controlled from the viewpoint ofthe structure of the metal mold.

In the present invention, an excessive amount of a molten resin isfilled based on the weight of an optically molded product in theinjection step. Right after the completion of filling, the cavity iscompressed to a prescribed center thickness in a short period of time.This compression time differs according to the type of the filling resinand molding conditions but it is preferably 5 seconds or less. When thetime is longer than 5 seconds, the molten resin is cooled and anextremely high compression pressure is required due to a rise in theviscosity of the resin.

The expression “prescribed thickness” as used here denotes a thicknesswithin a standard range of thickness which is typified from theviewpoint of the quality control of an optically molded product. Forexample, the thickness is the thickness of the center portion of a lensor the average thickness at a mirror surface of a lens.

The volume of the cavity is reduced by making the resin pressure lowerthan the pressure in the cavity resulting from compression after thecompletion of injection and compressing the cavity and an excess of themolten resin flows back into the injection cylinder from a gate througha runner and a sprue. This resin which has flown back is kneaded with aresin which is plasticized in the next cycle and re-injected.

After the completion of compression, the resin is cooled while theshrinkage amount of the molten resin is made up for by dwelling toobtain a molded product.

When the above first molding method is to be carried out, the inventorsof the present invention have found that an optically molded producthaving no optical distortion even when it is a lens having a largethickness difference and extremely small differences in quality amongproducts can be obtained by setting the resin pressure in the injectioncylinder to a range of 39 to 150 MPa, preferably 60 to 120 MPa and thenreducing the volume of the expanded cavity to a prescribed volume aftera molten thermoplastic resin is injected into the cavity through theinjection cylinder. That is, it is possible to mold a high-qualityoptically molded product stably by setting the resin pressure to theabove range.

In other words, in the above molding method, the molten thermoplasticresin is filled into the expanded cavity, the cavity is filled with theresin, the resin pressure obtained by calculating the hydraulic pressureof the injection cylinder from the ratio of the square of the diameterof the hydraulic injection cylinder to the square of the diameter of thescrew or the resin pressure measured with a pressure sensor reaches 39to 150 MPa as the predetermined pressure, and then the compression stepbegins. The resin pressure is preferably in the range of 60 to 120 MPa.When the injection resin pressure is less than 39 MPa, an opticaldistortion is easily produced in a molded product. The pressure set atthis point differs according to the shape of the optically moldedproduct, the shape of the metal mold, the resin used, the moldingmachine and the like.

When the cavity is expanded before injection to such a limit level ofthe above ratio that a weld line, an optical distortion or surfacedefect is rarely produced as described above, the molten resin is filledat the above resin pressure, and an excess of the resin is returned intothe injection cylinder, an optically molded product, for example, a lensalmost free from a weld line, optical distortion and surface defect canbe obtained even when it is a concave lens having a short focusingdistance. The return of the resin into the cylinder can be confirmedfrom an increase in the measurement value of an injection strokemeasuring instrument attached to an injection unit in a directionopposite to the injection direction of the resin.

As described above, the compression step begins after the resin pressurein the injection cylinder reaches a predetermined pressure beforecompression by setting the resin pressure to the above range. Therefore,a sufficient pressure is applied to the surface layer of a moldedproduct at the time of solidification, there are almost no differencesin pressure applied to the resin in the cavity before compression, andan optically molded product has no optical distortion and littledifferences.

A detailed description is subsequently given of the second moldingmethod of the present invention. The second molding method comprises thefollowing steps (1) to (6) as described above:

(1) expanding the volume of a cavity more than the volume of anoptically molded product of interest;

(2) injecting a molten thermoplastic resin into the cavity through aninjection cylinder;

(3) compressing the expanded cavity to a prescribed thickness of themolded product or a thickness 200 μm smaller than the thickness;

(4) adjusting or changing resin pressure in the injection cylinder andcompression pressure in the cavity in limits that the change width doesnot exceed 100 μm from the prescribed thickness of the molded product toachieve the prescribed thickness of the molded product in the end;

(5) keeping the molten thermoplastic resin in the cavity until themolded product of interest is formed; and

(6) taking out the obtained molded product from the cavity.

The second molding method is characterized in that the molten resininjected into the expanded cavity is compressed to a predeterminedthickness (or smaller than that) and then the resin pressure in theinjection cylinder is controlled or changed to a predetermined range asshown in the steps (2) to (4).

In the prior art, when an optically molded product obtains apredetermined thickness as shown in FIG. 4, counter force 11 isgenerated from a die set by contact between a compression control rod 4and the die set 5. Therefore, compression force 10 is not completelyapplied to the cavity 3 and a change in compression force 10 is absorbedas counter force 11 even when the compression force 10 is changed orcontrolled, whereby it is not reflected as the pressure of the cavity 3during molding. Therefore, pressure is not completely applied to aportion far from the gate or a thick portion, causing surfacedeformation. The concept of the second molding method of the presentinvention is shown in FIG. 5. In the second method, counter forceagainst the compression force 10 is generated not from a mold structuralpart but by the resin pressure 13 from the injection cylinder.Therefore, the compression pressure 10 is fully transmitted to thecavity, and when the compression pressure 10 is changed or controlled,it is directly reflected as the pressure of the cavity. In thecompression step, the resin pressure 13 is set to a level lower than thecompression pressure 10 to compress the cavity. Thereafter, after thetypical thickness of the cavity reaches a predetermined thickness of anoptically molded product, the typical thickness of the cavity iscontrolled within a prescribed range of a predetermined thickness of anoptically molded product in the end by controlling or changing the resinpressure and the compression pressure. The control of the pressure inthe cavity 3 by the compression pressure 10 in the dwelling step whichcannot be attained in the prior art is possible in the present inventionand sufficient compression pressure can be secured.

The expression “compression step” as used herein denotes a step in whichthe typical thickness of the expanded cavity is compressed to apredetermined thickness or a thickness 200 μm smaller than thepredetermined thickness, preferably a thickness 20 to 180 μm smallerthan the thickness. The expression “dwelling step” as used hereindenotes a step from the end of the compression step to the metering ofan injection molding machine.

Compression force of several tens to several hundreds of tons, whichdiffers according to the type, size and number per shot of opticallymolded products must be supported by the compression control rod in thecompression step and the dwelling step in the prior art. Therefore, thecompression control rod 4 must have sufficient buckling strength. Thecontact portions of the compression control rod 4 and the die set 5 musthave sufficient strength against elastic deformation. An ejector platemust have sufficient strength against flexure stress. Therefore, metalmold parts becomes large in size and a high-strength expensive materialis necessary. As a result, the metal mold becomes large in size andexpensive. Along with an increase in the size of the metal mold, alarger-sized molding machine is required, boosting costs. Since counterforce against the compression force 10 is generated by the resinpressure 13 in the second molding method of the present invention, thesufficient strengths of the movable mirror surface 2, pedestal 8,ejector plate 7 and compression plate 9 against compression stress mustbe taken into consideration. Therefore, excessive strength is notrequired, thereby making it possible to reduce the size of the metalmold. The calculation of the strength of the metal mold is easy.

As for the basic difference between the second method of the presentinvention and the method of the prior art, the space of the compressioncontrol rod 4 is made equal to the amount of compression in the priorart whereas the space is made larger than the amount of compression inthe second method of the present invention. Thereby, even when thecavity is compressed by a predetermined amount, the resin pressure 13 isapplied while the control rod and the die set on the fixed side are notcontacted to each other to be well balanced with the compressionpressure 10 to control the amount of compression, thereby controllingthe typical thickness of an optically molded product.

The expression “predetermined thickness” as used herein denote athickness within a prescribed range of typified thickness from theviewpoint of the quality control of an optically molded product. Thetypical thickness is the thickness of the center portion of a lens, thethickness of the peripheral portion of a lens or the average thicknessof a mirror surface and differs according to the shape of an opticallymolded product.

The expression “amount of compression” as used herein denotes adifference between the typical thickness of the cavity beforecompression and the typical thickness of the cavity during or aftercompression.

The expression “compression pressure” as used herein denotes a valueobtained by converting compression force generated from the compressionunit of a molding machine or metal mold into pressure to be received bythe cavity. As for how to calculate the value, the value is obtained bydividing compression force generated from the compression unit of amolding machine or a metal mold, by the projection area of a portionwhich is movable at the time of compression and contacted to the resinin the direction of the platen. The maximum compression force isdetermined by the designs of the compression units of a molding machineand a metal mold. The projection area of the portion which is movable atthe time of compression and contacted to the resin is the projectionarea in the direction of the platen of an optically molded product inthe case of the core compression method and a value obtained by addingthe projection area in the direction of the platen of the runner to theabove area in the case of the clamping compression method.

The expression “resin pressure” as used herein indicates a resinpressure obtained by calculating the hydraulic pressure of the injectioncylinder from the ratio of the square of the diameter of the hydraulicinjection cylinder to the square of the diameter of the screw or a resinpressure measured with a pressure sensor. This value differs accordingto the design of a molding machine.

Balancing between the resin pressure and the compression pressure meansthe control (change) of the resin pressure and the compression pressureso that the typical thickness of the cavity should fall within aprescribed range of thickness in the dwelling step (4) after thecompression step (3). Stated more specifically, in the case of examples,the compression pressure is set to 64.1 MPa when the resin pressure is63.3 MPa. The compression pressure is 0.8 MPa larger than the resinpressure. This is based on errors in the measurement accuracies ofresistance in the metal mold and molding machine, hydraulic pressure inthe molding machine and others. It is not necessary to make the resinpressure equal to the compression pressure but the typical thickness ofthe cavity (typical thickness of the optically molded product) may beset within a prescribed range of predetermined thickness. The resinpressure and the compression pressure to be well balanced with eachother differ according to the design of the metal mold, the shape andsize of the optically molded product, the type of molding machine andthe type of resin.

The second method will be described in detail with reference to FIG. 5.The structure of FIG. 5 is basically the same as that of FIG. 4 exceptthat a magnet scale 12 for the measurement of the amount of compressionfor control is attached to the ejector plate. The magnet scale 12 isinstalled to measure the moving amount of the ejector plate 7 but may beinstalled on the compression cylinder or the like. A rotary encoder,linear scale, micrometer, dial gauge, laser displacement gauge, infrareddisplacement gauge, limit switch or the like may be used in place of themagnet scale 12. Any means is acceptable if it detects the moving amount(the amount of compression) of the mirror surface of a movable mold.

The cavity is expanded more than the volume of an optically moldedproduct of interest. As for the expansion ratio of the cavity, thepercentage of the volume of the expanded cavity at the time of injectionto the volume of an optically molded product is preferably in the rangeof 110 to 500% in terms of expansion volume ratio calculated from theabove equation from the viewpoints of the surface accuracy (surfacedeformation or etc.), optical properties (focusing distance, aberration,etc.) and molding ease of an optically molded product. The expansionvolume ratio is particularly preferably in the range of 110 to 400%.When the expansion volume ratio is more than 500%, the amount of theexhausted resin becomes large, whereby required compression pressure maybecome high, the heat resistance of the molten resin may deteriorate, ora molding failure may occur. The expression “amount of compression”caused by this compression denotes a difference in typical thicknessbetween an optically molded product before compression and an opticallymolded product after compression.

It is important to expand the cavity in limits that a weld line, opticaldistortion or surface defect is not formed or is allowable. It isdifficult to completely eliminate a weld line, optical distortion orsurface defect which is formed before compression by any means.

The compression step begins after the resin is injected into the cavityin the injection step.

As shown in FIG. 6, the cavity is compressed to a predeterminedthickness by setting the resin pressure 20 to a value smaller than thecompression pressure 21 in the compression step 19. The typicalthickness of the cavity (typical thickness of an optically moldedproduct) is compressed to a predetermined thickness by the time of thiscompression step, the resin pressure and the compression pressure. Thetime of the compression step which differs according to the type offilling resin and molding conditions is preferably 5 seconds or less.When this time is longer than 5 seconds, the molten resin is cooled withthe result of an increase in viscosity, thereby requiring an extremelyhigh compression pressure. A distortion may remain in the thin portionof an optically molded product due to this high compression pressure.

Thereafter, the resin pressure 22 and the compression pressure 23 arewell balanced to proceed to the dwelling step 37. At this point, amirror surface 2 on a movable side may contact a mirror surface 1 on afixed side due to the relationship among the resin pressure 22, thecompression pressure 23 and the time 19 of the compression step. Thistakes place, for example, when the time 19 of the compression step istoo long or when the compression pressure 21 is large. When these mirrorsurfaces contact each other, they are broken and cannot be used. Toavoid contact between the mirror surfaces, it is desired to set thespace 6 between the compression control rod and the die set to a value0.2 to 0.6 mm smaller than the thickness of the obtained product.

In the compression step 19, as shown in FIG. 7, it is desirable that thecavity should be compressed excessively to a value 200 μm (preferably 20to 180 μm) smaller than the predetermined thickness and returned to thepredetermined thickness by controlling the resin pressure 20, the resinpressure time 18 in the compression step, the compression pressure 21and the compression pressure time 17 in the compression step. As anillustrative example, the time 17 for reducing the compression pressureand the time 18 for raising the resin pressure maybe delayed to setthese values. By returning the cavity to the predetermined thickness, anexcessive pressure applied to the thin portion of the molded product isreleased, whereby a uniform pressure is applied to the whole moldedproduct. When the amount of return from compression 25 is in the rangeof 20 to 200 μm, surface accuracy improves and optical distortiondecreases. When the amount is 200 μm or more, molding stability such asthe stability of lens diopter deteriorates.

As described above, in the second method of the present invention, thecompression pressure can be controlled in the dwelling step as well. InFIG. 6 and FIG. 7, the resin pressure 22 in the dwelling step is hardlytransmitted to the inside of the cavity because the sprue and the gateare gradually solidified by the progress of cooling, the compressionpressure 23 exceeds the resin pressure 22, and the amount of compressiongradually increases to width 24. The amount of compression graduallyalso increases to width 24 by the shrinkage of the resin. A time changein the amount of compression causes variations of curvature radius in athin portion which has already been solidified and a thick portion whichis being solidified.

As shown in FIG. 8, the resin pressure 27 and the compression pressure28 are balanced in multiple stages along with the progress of cooling tocontrol a change in the typical thickness of the cavity, that is, achange width 24 in the amount of compression (a change in the typicalthickness of the cavity will be referred to as “a change width in theamount of compression” hereinafter) to 100 μm or less, preferably 50 μmor less. This makes it possible to achieve a uniform curvature radiusfrom the center portion to the peripheral portion of the molded product.Describing how to set the change width, the measurement value of acompression measuring instrument when the typical thickness of thecavity reaches a predetermined thickness is taken as 0. When the amountof compression increases and the typical thickness of the cavitydecreases, the resin pressure is increased or the compression pressureis reduced. When the amount of compression decreases and the typicalthickness of the cavity increases, the resin pressure is reduced or thecompression pressure is increased. The measurement of the amount ofcompression is carried out with a compression measuring instrument suchas the above magnet scale. To simplify this setting, a loop circuit ispreferably used to feedback the amount of displacement of the magnetscale or the like to the setting of the resin pressure and thecompression pressure of the molding machine.

EXAMPLES

The following examples are given to further illustrate the presentinvention.

The evaluation items and evaluation methods of an optically moldedproduct in examples and comparative examples will be describedhereinunder.

(1) Refractive Power and Curvature Radius

The measurement of curvature radius is evaluated using the OMS-401 Moirélaser interferometer of Rotlex Co., Ltd. The refractive power isevaluated by converting the mirror surface of a metal mold used and thecurvature radius of an optically molded product (lens in this case) intorefractive power diopter at a refractive index of 1.586 from thefollowing equation. The smaller the difference in refractive powerbetween the mirror surface and the optically molded product, the betterthe obtained optically molded product becomes.

refractive power=586/(curvature radius)

The unit of curvature radius is mm.

(2) Surface Accuracy

The surface accuracy is evaluated using the OMS-401 Moiré laserinterferometer of Rotlex Co., Ltd. The surface accuracy is evaluatedbased on the following five criteria.

5 The shift of a Moiré interference fringe is not observed.

4 The shift of a Moiré interference fringe is 25% or less of an intervalbetween interference fringes.

3 The shift of a Moiré interference fringe is 50% or less of an intervalbetween interference fringes.

2 The shift of a Moiré interference fringe is 100% or less of aninterval between interference fringes.

1 The shift of a Moiré interference fringe is more than 100% of aninterval between interference fringes.

(3) Observation of Fluorescent Lamp

The observation of a fluorescent lamp is carried out by keeping a lens30 cm away from the eye in a downward direction 29 as shown in FIG. 9and observing an image reflected from a straight pipe 30 W fluorescentlamp 30 located about 15 cm away from the eye and above the lens atalmost the same height as the eye. The image is evaluated based on thefollowing 5 criteria.

5 The image of the fluorescent lamp is a smooth and uniform curve.

4 The image of the fluorescent lamp is smooth but the curvature radiuschanges at 2 places or less.

3 The image of the fluorescent lamp is smooth but the curvature radiuschanges at 4 places or less.

2 The image of the fluorescent lamp bends at 2 places or less.

1 The image of the fluorescent lamp bends at more than 2 places.

(4) Observation of Depolarizing Plate (Optical Distortion and Weld Line)

An optical distortion and weld line are evaluated and measured for theirlengths using a polariscope (PS-5 of Riken Keiki Co., Ltd.). As formeasurement with a polariscope, light from a fluorescent lamp 34 (30 Wannular fluorescent lamp) diverged by frosted glass 33 is evaluated byplacing an optically molded product 32 between two polarizing plates 31whose interval is about 15 cm and whose polarization planes are parallelto each other as shown in FIG. 10. The evaluation is carried out basedon the following criteria.

5 No interference fringes are existent in the used portion of the lens.

4 An interference fringe which is shifted 0.5 the wavelength is observedin the used portion of the lens.

3 One interference fringe which is shifted 1 the wavelength is observedin the used portion of the lens.

2 Two interference fringes which are shifted 1 the wavelength areobserved in the used portion of the lens.

1 More than two interference fringes which are shifted 1 the wavelengthare observed in the used portion of the lens.

The used portion of the lens described above denotes a portion of 70 mmfrom the center of the lens having an outer diameter of 77.5 mm in theexamples. The length 36 of the weld line shown in FIG. 11 is measuredwith a caliper (CS-S15M of Mitsutoyo Co., Ltd.).

(5) Amount of Compression

The amount of compression is measured with a magnet scale (LH-20B ofSony Corporation) installed on the molding compression cylinder.

Example 1

0.3 part by weight of 2-(2′-hydroxy-5′-t-octyl)benzotriazole as anultraviolet light absorber, 0.03 part by weight oftris(nonylphenyl)phosphite as a heat stabilizer and 0.2 part by weightof monoglyceride stearate as a releasing agent were blended with 100parts by weight of a polycarbonate resin having a viscosity averagemolecular weight of 22,500 synthesized from bisphenol A and phosgene,and the obtained blend was formed into the following polycarbonate resinminus spectacle lens (concave lens) by injection compression moldingusing the injection molding machine (SYCAPSG220) of Sumitomo HeavyIndustries, Ltd. and a core compression mold.

curvature radius of front side 293.00 mm curvature radius of rear side−73.25 mm thickness of center portion 1.5 mm edge thickness 10.0 mmouter diameter of lens 77.5 mm focusing distance at apex of rear side−166.67 mm

The main molding conditions at this point were as follows.

temperature of cylinder 280° C. to 300° C. temperature of metal mold125° C. molding cycle 240 seconds

The movable lens mold was moved back, the cavity was expanded to a lenscenter thickness of 7.6 mm (expansion volume ratio of 215%) beforeinjection, the resin was injected into the cavity, the movable lens moldwas compressed when the resin pressure became 30 MPa until thecompression control rod contacted the die set and the lens centerthickness became 1.5 mm as shown in FIG. 5, and an excess of the resinwas returned into the injection cylinder. The return of an excess of theresin into the cylinder was confirmed from an increase in themeasurement value of an injection stroke measuring instrument in adirection opposite to that at the time of injection. Thereafter, aconcave lens molded product was taken out after the completion ofcooling. No weld line was observed in the obtained concave lens whenmeasured with a polariscope.

Comparative Example 1

A similar polycarbonate resin minus spectacle lens (concave lens) wasformed by compression molding under the same conditions as in Example 1except that the cavity had a lens center thickness of 1.6 mm beforeinjection and an excess of the resin was compressed without returninginto the injection cylinder. The obtained concave lens molded producthad a weld line of 25 to 35 mm and low quality as a lens.

Example 2

0.3 part by weight of 2-(2′-hydroxy-5′-t-octyl)benzotriazole as anultraviolet light absorber, 0.03 part by weight oftris(nonylphenyl)phosphite as a heat stabilizer and 0.2 part by weightof monoglyceride stearate as a releasing agent were blended with 100parts by weight of a polycarbonate resin having a viscosity averagemolecular weight of 22,500 synthesized from bisphenol A and phosgene,and the obtained blend was formed into the following spectacle concavelens by injection compression molding using the injection moldingmachine (SYCAPSG220) of Sumitomo Heavy Industries, Ltd. and a corecompression mold.

curvature radius of front side 293.00 mm curvature radius of rear side−73.25 mm thickness of center portion 1.5 mm thickness of peripheralportion 10.0 mm outer diameter of lens 77.5 mm focusing distance at apexof rear side −166.67 mm

The main molding conditions at this point were as follows.

temperature of cylinder 280° C. to 300° C. temperature of metal mold125° C. molding cycle 240 seconds resin pressure before compression56.84 MPa

The movable lens mold was moved back, the cavity was expanded to a lenscenter thickness of 7.6 mm (expansion volume ratio of about 215%) beforeinjection, the resin was injected into the cavity, the movable lens moldwas compressed when the resin pressure became 56.8 MPa until thecompression control rod contacted the die set and the lens centerthickness became 1.5 mm as shown in FIG. 5, and an excess of the resinwas returned into the injection cylinder. The return of an excess of theresin into the cylinder was confirmed from an increase in themeasurement value of an injection stroke measuring instrument in adirection opposite to that at the time of injection. Thereafter, apolycarbonate resin minus spectacle lens (concave lens) molded productwas taken out after the completion of cooling.

This was a high-accuracy spectacle lens having no such defects as asurface defect, optical distortion and weld line when it was observedwith a polarizing plate. The standard deviation of lens refractive powerdiopter was satisfactory at 0.003.

Comparative Example 2

A similar concave lens was formed by compression molding under the sameconditions as in Example 2 except that the cavity had a lens centerthickness of 1.6 mm before injection and an excess of the resin wascompressed without returning into the injection cylinder. Line disorderwas seen in an image reflected from a fluorescent lamp and an opticaldistortion was observed in the peripheral portion of the lens. A 25 to35 mm long weld line was confirmed. The standard deviation of refractivepower diopter was 0.062. This lens was inferior in quality.

Example 3

Molding was carried out under the same conditions as in Example 2 exceptthat a polycarbonate resin plus spectacle lens (convex lens) having thefollowing specifications was molded.

curvature radius of front surface 97.67 mm curvature radius of rearsurface −146.50 mm thickness of center portion 3.7 mm edge thickness 1.0mm outer diameter of lens 77.5 mm focusing distance at apex of rear side500.0 mm

The movable lens mold was moved back, the cavity was expanded to a lenscenter thickness of 5.1 mm (expansion volume ratio of about 160%) beforeinjection, the resin was injected into the cavity, the movable lens moldwas compressed when the resin pressure became 56.8 MPa until thecompression control rod contacted the die set and the lens centerthickness became 3.7 mm as shown in FIG. 5, and an excess of the resinwas returned into the injection cylinder. The return of an excess of theresin into the cylinder was confirmed from an increase in themeasurement value of an injection stroke measuring instrument in adirection opposite to that at the time of injection.

Thereafter, a convex lens molded product was taken out after it wascooled while dwelling. Such defects as the optical distortion andsurface defect of the center portion of the lens were not observed inthe obtained convex lens with a polarizing plate. The standard deviationof refractive power diopter was satisfactory at 0.004. Thus, ahigh-accuracy convex lens was obtained.

Comparative Example 3

A similar convex lens was formed by compression molding under the sameconditions as in Example 3 except that the cavity had a lens centerthickness of 3.8 mm before injection and an excess of the resin wascompressed without returning into the injection cylinder.

A large distortion and a pattern based on the flow line of the resinwere observed in a portion near the gate and an optical distortion wasobserved in the center portion when the obtained convex lens wasobserved with a polarizing plate. The standard deviation of refractivepower diopter was 0.072. Thus, the obtained lens had quality problems.

Example 4

0.3 part by weight of 2-(2′-hydroxy-5′-t-octyl)benzotriazole as anultraviolet light absorber, 0.03 part by weight oftris(nonylphenyl)phosphite as a heat stabilizer and 0.2 part by weightof monoglyceride stearate as a releasing agent were blended with 100parts by weight of a polycarbonate resin having a viscosity averagemolecular weight of 22,500 synthesized from bisphenol A and phosgene,and the obtained blend was formed into the following spectacle concavelens by injection compression molding using the injection moldingmachine (SYCAPSG220) of Sumitomo Heavy Industries, Ltd. and a corecompression mold.

curvature radius of front side 293.0 mm curvature radius of rear side−73.25 mm thickness of center portion 1.5 mm edge thickness 10.0 mmouter diameter of lens 77.5 mm focusing distance at apex of rear side−166.67 mm

The main molding conditions at this point were as follows.

temperature of cylinder 280° C. to 300° C. temperature of metal mold125° C. molding cycle 240 seconds

The movable lens mold was moved back, the cavity was expanded to a lenscenter thickness of 7.6 mm (expansion volume ratio of about 215%) beforeinjection, and the resin was injected into the cavity and molded by themethod shown in FIG. 6 when the resin pressure became 56.8 MPa. In thecompression step, the movable lens mold was compressed to a lens centerthickness of 1.5 mm and an excess of the resin was returned into theinjection cylinder. At this point, the compression control rod and thedie set were not contacted to each other. This was confirmed by checkingno red chalk adhered to the die set after the red chalk was applied tothe end of the compression control rod. The return of an excess of theresin into the cylinder was confirmed from an increase in themeasurement value of the injection stroke measuring instrument in adirection opposite to that at the time of injection. The resin pressurein the compression step was 12.4 MPa, its application time was 2.14seconds, the compression pressure was 102.56 MPa and its applicationtime was 2.14 seconds. Thereafter, the resin pressure was set to 63.3MPa and the compression pressure was set to 64.1 MPa in the dwellingstep to balance both pressures. The amount of compression was changed to165 μm (change width in the amount of compression) in the dwelling stepin a direction that an optically molded product became thin. Thereafter,a polycarbonate resin minus spectacle lens (concave lens) was taken outafter the completion of cooling. The evaluation results of the obtainedpolycarbonate resin minus spectacle lens are shown in Table 1.

Comparative Example 4

The metal mold used in Example 4 was used, the compression amountcontrol space was set to 1 mm, and the movable mold was opened to a lenscenter thickness of 2.5 mm which was 1 mm larger than the predeterminedlens center thickness of 1.5 mm before injection. In the injection step,a resin was injected in an amount of 100 to 105% the volume of thecavity. In this case, the resin did not return into the cylinder. Thiswas confirmed from not increasing in the measurement value of theinjection stroke measuring instrument in a direction opposite to that atthe time of injection. Thereafter, the cavity was compressed by applyinga compression force of 102.56 MPa and contacting the compression controlrod to the die set on the fixed side. Contact between the compressioncontrol rod and the die set was confirmed by checking the adhesion ofred chalk to the die set after the red chalk was applied to the end ofthe compression rod.

A similar polycarbonate resin minus spectacle lens (concave lens) wasformed by compression molding under the same conditions as in Example 4except the above. The evaluation results of the obtained polycarbonateresin minus spectacle lenses are shown in Table 1.

Example 5

The movable lens mold was compressed by 100 μm (amount of return fromcompression) more than a lens center thickness of 1.5 mm and an excessof the resin was returned into the injection cylinder in the compressionstep of Example 4. At this point, the compression control rod and thedie set were not contacted to each other. The resin pressure in thecompression step was 12.4 MPa, its application time was 2.10 seconds,the compression pressure was 102.56 MPa and its application time was2.16 seconds. Thereafter, the resin pressure was set to 63.3 MPa and thecompression pressure was set to 64.1 MPa in the dwelling step to balanceboth pressures. The amount of compression was changed by 155 μm (changewidth in the amount of compression) in a direction that an opticallymolded product became thin in the dwelling step. Thereafter, apolycarbonate resin minus spectacle lens (concave lens) was taken outafter the completion of cooling.

A similar polycarbonate resin minus spectacle lens was formed bycompression molding under the same conditions as in Example 4 except theabove. The evaluation results of the obtained polycarbonate resin minusspectacle lenses are shown in Table 1.

Example 6

The movable lens mold was compressed by 100 μm (amount of return fromcompression) more than a lens center thickness of 1.5 mm and an excessof the resin was returned into the injection cylinder in the compressionstep of Example 4. At this point, the compression control rod and thedie set were not contacted to each other. The resin pressure in thecompression step was 12.4 MPa, its application time was 2.10 seconds,the compression pressure was 102.56 MPa and its application time was2.16 seconds. Thereafter, the resin pressure was set to 63.3 MPa and thecompression pressure was set to 64.1 MPa in the dwelling step to balanceboth pressures. The resin pressure was gradually increased from 63.3 MPato 68.7 MPa stepwise at intervals of 2 to 90 seconds. The compressionpressure was balanced with the resin pressure and then gradually reducedfrom 64.1 MPa to 42.5 MPa stepwise at intervals of 2 to 60 seconds. Theamount of compression was changed by 35 μm (change width in the amountof compression) in the dwelling step. A polycarbonate resin minusspectacle lens (concave lens) was taken out after the completion ofcooling.

A similar polycarbonate resin minus spectacle lens was formed bycompression molding under the same conditions as in Example 4 except theabove. The evaluation results of the obtained polycarbonate resin minusspectacle lenses are shown in Table 1.

Example 7

0.3 part by weight of 2-(2′-hydroxy-5′-t-octyl)benzotriazole as anultraviolet light absorber, 0.03 part by weight oftris(nonylphenyl)phosphite as a heat stabilizer and 0.2 part by weightof monoglyceride stearate as a releasing agent were blended with 100parts by weight of a polycarbonate resin having a viscosity averagemolecular weight of 22,500 synthesized from bisphenol A and phosgene,and the obtained blend was formed into following spectacle convex lensby injection compression molding using the injection molding machine(SYCAPSG220) of Sumitomo Heavy Industries, Ltd. and a core compressionmold.

curvature radius of front side 97.67 mm curvature radius of rear side−146.50 mm thickness of center portion 3.7 mm edge thickness 1.0 mmouter diameter of lens 77.5 mm focusing distance at apex of rear side500.0 mm injected resin pressure before compression 56.8 MPa

The movable lens mold was moved back, the cavity was expanded to a lenscenter thickness of 5.1 mm (expansion volume ratio of about 160%) beforeinjection, and the resin was injected into the cavity and molded by themethod shown in FIG. 6 when the resin pressure became 56.8 MPa. In thecompression step, the movable lens mold was compressed to a lens centerthickness of 3.7 mm and an excess of the resin was returned into theinjection cylinder. At this point, the compression control rod and thedie set were not contacted to each other. This was confirmed by checkingno red chalk adhered to the die set after the red chalk was applied tothe end of the compression control rod. The return of an excess of theresin into the cylinder was confirmed from an increase in themeasurement value of the injection stroke measuring instrument in adirection opposite to that at the time of injection. The resin pressurein the compression step was 18.6 MPa, its application time was 0.42seconds, the compression pressure was 102.56 MPa and its applicationtime was 0.42 seconds. Thereafter, the resin pressure was set to 63.3MPa and the compression pressure was set to 64.1 MPa in the dwellingstep to balance both pressures. Thereafter, a polycarbonate resin plusspectacle lens (convex lens) was taken out after the completion ofcooling. The amount of compression was changed to 135 μm (change widthin the amount of compression) in the dwelling step in a direction thatan optically molded product became thin. The evaluation results of theobtained polycarbonate resin plus spectacle lens are shown in Table 1.

Comparative Example 5

The metal mold used in Example 4 was used, the compression amountcontrol space was set to 1 mm, and the movable mold was opened to a lenscenter thickness of 4.7 mm which was 1 mm larger than the predeterminedlens center thickness of 3.7 mm before injection. In the injection step,a resin was injected in an amount of 100 to 105% the volume of thecavity. In this case, the resin did not return into the cylinder. Thiswas confirmed from not increasing in the measurement value of theinjection stroke measuring instrument in a direction opposite to that atthe time of injection. Thereafter, the cavity was compressed by applyinga compression force of 102.56 MPa and contacting the compression controlrod to the die set on the fixed side. Contact between the compressioncontrol rod and the die set was confirmed by checking the adhesion ofred chalk to the die set after the red chalk was applied to the end ofthe compression rod.

A similar polycarbonate resin plus spectacle lens (convex lens) wasformed by compression molding under the same conditions as in Example 7except the above. The evaluation results of the obtained polycarbonateresin plus spectacle lenses are shown in Table 1.

Example 8

The movable lens mold was compressed by 100 μm (amount of return fromcompression) more than a lens center thickness of 3.7 mm and an excessof the resin was returned into the injection cylinder in the compressionstep of Example 7. At this point, the compression control rod and thedie set were not contacted to each other. The resin pressure in thecompression step was 18.6 MPa, its application time was 0.39 seconds,the compression pressure was 102.56 MPa and its application time was0.44 seconds. Thereafter, the resin pressure was set to 63.3 MPa and thecompression pressure was set to 64.1 MPa in the dwelling step to balanceboth pressures. The amount of compression was changed by 145 μm (changewidth in the amount of compression) in a direction that an opticallymolded product became thin in the dwelling step. Thereafter, apolycarbonate resin plus spectacle lens (convex lens) was taken outafter the completion of cooling. The evaluation results of the obtainedpolycarbonate resin plus spectacle lens are shown in Table 1.

Example 9

The movable lens mold was compressed by 100 μm (amount of return fromcompression) more than a lens center thickness of 3.7 mm and an excessof the resin was returned into the injection cylinder in the compressionstep of Example 7. At this point, the compression control rod and thedie set were not contacted to each other. The resin pressure in thecompression step was 18.6 MPa, its application time was 0.39 seconds,the compression pressure was 102.56 MPa and its application time was0.44 seconds. Thereafter, the resin pressure was set to 63.3 MPa and thecompression pressure was set to 64.1 MPa in the dwelling step to balanceboth pressures. The resin pressure was gradually increased from 63.3 MPato 68.7 MPa stepwise at intervals of 2 to 90 seconds. The compressionpressure was balanced with the resin pressure and then gradually reducedfrom 64.1 MPa to 42.5 MPa stepwise at intervals of 2 to 60 seconds. Theamount of compression was changed by 35 μm (change width in the amountof compression) in a direction that an optically molded product becamethin in the dwelling step. A polycarbonate resin plus spectacle lens(convex lens) was taken out after the completion of cooling. Theevaluation results of the obtained polycarbonate resin plus spectaclelens are shown in Table 1.

TABLE 1 evaluation of Moiré laser interferometer curvature radius (mm)refractive power mold mirror mold mirror surface lens surfaceobservation of observation with mov- mov- mov- lens surface accuracyfluorescent lamp polarizing plate fixed able fixed able fixed able fixedmovable fixed movable fixed movable dis- weld side side side side sideside side side side side side side tortion line Ex. 4 293.00 73.25290.10 72.89 2.00 8.00 2.02 8.04 4 4 3 4 3 non- existent Ex. 5 293.0073.25 291.54 73.16 2.00 8.00 2.01 8.01 5 5 4 5 4 non- existent Ex. 6293.00 73.25 291.54 73.25 2.00 8.00 2.01 8.00 5 5 5 5 4 non- existent C.Ex. 4 293.00 73.25 285.85 72.61 2.00 8.00 2.05 8.07 1 2 1 3 1 20 to 30mm Ex. 7 97.67 146.50 97.99 145.05 6.00 4.00 5.98 4.04 4 4 3 3 4 non-existent Ex. 8 97.67 146.50 97.67 145.77 6.00 4.00 6.00 4.02 5 4 3 4 4non- existent Ex. 9 97.67 146.50 97.67 146.13 6.00 4.00 6.00 4.01 5 5 55 5 non- existent C. Ex. 5 97.67 146.50 98.49 143.98 6.00 4.00 5.95 4.071 1 1 1 2 non- existent Ex.: Example C. Ex.: Comparative Example

Effect of the Invention

An optically molded product free from a weld line can be produced by theinjection compression molding method of the present invention using ametal mold having an extremely simple structure and a method of moldingthereof. Therefore, the method of the present invention isadvantageously used for the injection compression molding of a spectaclelens, particularly useful for the injection compression molding of apolycarbonate resin minus spectacle lens.

An optically molded product free from an optical distortion andexcellent in surface accuracy can be produced stably by the moldingmethod of the present invention.

The molding method of the present invention makes it possible to producemultiple types and a great quantity of spectacle lenses at the same timewith a simple apparatus.

What is claimed is:
 1. A method of producing an optically molded productof a thermoplastic resin by injection compression molding, the methodcomprising the steps: (1) expanding the volume of a cavity more than thevolume of the optically molded product of interest; (2) injecting amolten thermoplastic resin into the cavity through an injectioncylinder; (3) compressing the expanded cavity to a prescribed thicknessof a center portion of the molded product or a thickness 200 μm smallerthan the prescribed thickness; (4) changing resin pressure in theinjection cylinder and compression pressure in the cavity in limits thatthe change width does not exceed 100 μm from the prescribed thickness ofcenter portion of the molded product to achieve the prescribed thicknessof the molded product in the end; (5) keeping the molten thermoplasticresin in the cavity until the molded product of interest is formed; and(6) taking out the obtained molded product from the cavity.
 2. Themolding method of claim 1, wherein the expanded cavity is compressed toa thickness 20 to 180 μm smaller than a prescribed thickness of centerportion of molded product in the step (3).
 3. The molding method ofclaim 1, wherein a value of the expansion volume ratio (%) which isobtained from [(volume of extended cavity)/(volume of molded product ofinterest)]×100 is in the range of 110 to 500% in the step (1).
 4. Themolding method of claim 1, wherein the thermoplastic resin is anaromatic polycarbonate resin.
 5. The molding method of claim 1, whereinthe optically molded product is a spectacle lens.